For the manufacturing of the next generation of integrated circuit (IC) computer chips, the standard lithographic methods are not adequate. Reducing structure size is not feasible because of quantum effects and interference between the single conducting elements. Thus, the electronic industry has devised so-called 3-D chips, or flip chips. These consist of several IC chips stacked together. The reduced distance between the elements allows for more compact, more complex, and faster processors. Market growth in such chips is expected to be exponential, as more and more consumer electronics require smaller components with higher processing power, e.g. computers, cell phones, and other portable consumer electronic devices.
These stacked elements have to be soldered together, within very demanding tolerances and in such a way as to not impair the function of the IC element. The soldering is achieved by using tin (Sn) or lead (Pb) compounds as solders. The reduced distance between stacked IC chips, however, pose considerable risk that a soft error may occur due to the influence of alpha rays from materials in the vicinity of the IC chip. Accordingly, the solder material and additional functional layers deposited on the electronic devices before or after soldering must be “low alpha”, as it is known in the art, meaning that it does not emit alpha radiation (technically, charged helium nuclei, He2+). This means ensuring that, in the case of Sn, very high purity Sn and Sn compounds must be used, with no Pb contamination since Pb has an isotope which decays through Polonium, which is an alpha emitter. Other typical alpha emitting contaminants are uranium and thorium, which also must be reduced to a minimum content.
Stannous oxide (SnO) is a tin compound used, for example, during the fabrication of IC chips. As disclosed in US 2010/0116674 and patents cited within, electronic devices are electroplated with Sn or Sn-based alloys. Acid solutions of tin(II) compounds are used for the electroplating process. The amount of Sn deposited on the component parts needs to be replenished regularly or continuously to allow constant operation without quality variation. US 2010/0116674 teaches why SnO is the best source of Sn(II) for that application.
One previously reported method of preparing stannous oxide comprises reacting a stannous salt acidic aqueous solution and an alkali hydroxide solution at a pH of 11-12.5. An alkali carbonate is then added to the mixture to yield stannous oxide. See JP 3223112 A. However, carrying out the reaction of the alkali hydroxide with the stannous salt acidic in the aqueous results in impurities, such as the anions of the stannous salt, residing within the final product. In addition, the high pH levels produce low purity stannous oxide, as does the addition of the alkali carbonate, which introduces contamination by foreign cations.
Accordingly, for certain applications such as in electroplating stacked IC chips, the SnO must be highly pure, which includes being essentially free of corrosive anions, such as halide, essentially free of trace metal impurities, and essentially free of alpha-radiation emitters, as discussed above. Further, it is essential to have a low concentration of stannic oxide (SnO2), since SnO2 does not dissolve in most of the acids used in the electroplating baths. Instead, any stannic oxide present forms sludge in the bath, which requires a potentially troublesome mechanical means for removal. The present invention addresses these needs, among others.